1. Field of the Invention
The present invention generally relates to the field of optical fibers, more particularly to microstructured optical fibers.
2. Description of the Related Art
Optical fibers are largely used in optical telecommunication systems.
In several applications, fiber transmission loss is a critical aspect. For example, fiber loss is a major constraint from which the structure of a long-haul optical transmission system and the related costs depend.
Silica-based transmission fibers exhibit a typical transmission loss of about 0.2 dB/km in a wide spectral window of several tens of nanometers, centered at about 1550 nm. Due to fiber loss (and also to material's intrinsic nonlinearities) the maximum fiber span length is typically not higher than 100 km, and the power loss in such length can be of over 20 dB.
The resulting segmentation of a long-haul link into shorter intermediate spans contributes to the complexity and costs of the overall system. Thus, even a minor reduction of the fiber loss would result into a great overall economic advantage. Benefits could follow even in applications involving shorter links, where single spans are needed, because requirements on the optical components at the link termination would be relaxed thanks to an increase in the power budget.
Currently, silica-based transmission fibers typically include germanium doping in the fiber core, so as to achieve the desired refractive index difference between the core and cladding.
Several studies have shown that in low-loss, silica-based fibers, the loss results from four main contributions: the Rayleigh scattering from density fluctuations in the core material, the Rayleigh scattering from concentration fluctuations of the dopants, the loss resulting from waveguide imperfections, and the infrared absorption loss. These conclusions are for example reported in S. Sakaguchi et al., ‘Rayleigh scattering in silica glass with heat treatment’, Journal of Non-Crystalline Solids 220, 1997, pp. 178-186; K. Tsujikawa et al., ‘Rayleigh Scattering Reduction Method for Silica-Based Optical Fiber’, Journal of Lightwave Technology 18, pp. 1528-1532; R. Le Parc et al., ‘Thermal annealing and density fluctuations in silica glass’, Journal of Non-Crystalline Solids 293-295, 2001, pp. 295 366; and in the U.S. Pat. No. 6,404,965. The first and third above-cited contributions to fiber loss can be optimized through an appropriate profile design and through the optimization of the fiber drawing process. In particular, the preform drawing speed and temperature, the drawing tension and the annealing furnace temperature can reduce the glass fixation temperature and therefore reduce the first and third contributions to fiber loss cited above.
However, the Rayleigh scattering loss resulting from fluctuations of the concentration of the dopants cannot be avoided by any of these optimization techniques. In particular, as a relevant amount of GeO2 is used in the fibers to provide the necessary up-doping in the fiber core, typically a loss of at least 0.02 dB/km is to be ascribed to the presence of the dopant, as reported in Sakaguchi et al., ‘Optical properties of GeO2 glass and optical fibers’, Applied Optics, Vol. 36, 1997, p. 6809-6814.
Pure silica core (i.e., undoped core) fibers can potentially feature a lower loss than fibers with germanium-doped core. In a pure silica core fiber, a doped cladding of lower refractive index than the core has to be provided in order to confine light in the core. Since only a small fraction of the transmitted light travels in the cladding, the pure silica core fibers can potentially maintain the low loss of pure silica.
A known solution for making a pure silica core is to dope the silica cladding with fluorine so as to achieve a suitable refractive index profile. However, the Applicant observes that a drawback of this type of fibers is that the whole cladding has to be doped with fluorine, and the manufacturing process is therefore more expensive, since the volume of material to be doped is much greater than in the core.
A third solution which avoids either up-doping of the fiber core or low-doping of the cladding is provided by microstructured fibers, i.e. fibers having a microstructured cladding wherein holes, typically filled with air (generally, with a material having a lower refractive index than silica) run along the fiber parallel to the fiber axis. A brief review of these fibers, also known as “Photonic Crystal Fibers” (PCFs) or “Photonic BandGap fibers” (PBG) or “holey fibers”, is provided in A. Bjarklev et al., ‘Photonic crystal fibres—The state-of-the-art’, 28th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002.
Microstructured fibers can be manufactured in several different ways. One method, known as “stack-and-draw”, includes stacking silica capillary tubes inside a hollow glass cylinder in a close-packed space arrangement, welding the tubes together and then drawing the resulting preform by a conventional fiber preform drawing method.
Although microstructured claddings may be combined with an up-doped core, typically the microstructured fibers have a core made of a low-loss, pure (i.e., not doped) material, such as pure silica, the guiding of light in the core being ensured by the provision of the microstructured cladding. In fact, the pattern of holes, typically air-filled, leads to an effective lowering of the refractive index. Potentially, the loss in these fibers could benefit from all the optimizations of the drawing conditions previously discussed, without any penalty related to relevant quantities of chemical dopants which are instead used in conventional fibers, including the low-doped cladding, pure silica core fibers.
However, microstructured fibers proposed so far are still affected by loss problems.
In L. Farr et al., ‘Low loss photonic crystal fibers’, 28th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002, a loss of 0.58 dB/km at 1550 nm in a microstructured fiber having a pure, undoped silica core, with an outer diameter of 170 μm, a pitch A between the holes (holes spacing) of 4.2 μm, an air hole diameter d of 1.85 μm and at least four shells of holes or sixty holes as shown in FIGS. 1 (a) and 1 (b) has been reported. Such a loss value is much larger than what is obtained in conventional (i.e., non-microstructured) fibers. The excess loss has been ascribed to absorption from the hydroxyl group, from metal impurities, and from a remarkable increase in the Rayleigh scattering.
K. Tajma et al., ‘Low-loss photonic crystal fibers’, OFC 2002, ThS3, p. 523-524, compares two types of PCF: a first type having conventional core and cladding structure (GeO2-doped silica core and pure silica cladding) and six holes on one ring around the core to shift the zero-dispersion wavelength, and a second type made of pure silica glass and having sixty holes in the cladding.
Both of the structures of Tajma et al. have loss problems. The GeO2-doped core fiber exhibits the Rayleigh scattering problems previously mentioned, while the fiber with a high number of holes has high Rayleigh scattering contributions from silica-air hole imperfections (such as roughness of the hole surfaces) and from OH contamination. In addition to the contamination and surface scattering problems, which are detrimental for the fiber loss, a fiber with a large number of holes is also more difficult to manufacture, due to the complexity in making the holes, and potentially suffers from mechanical strength problems both during the manufacturing process and during its use, due to the large empty volume in its structure. In particular, the final shape of the holes resulting from the fiber preform drawing, the strength of the fiber and the splicing properties are all adversely related to their total number.
Λ microstructured fiber with a smaller number of holes has been described in D. Asnaghi et al. “Fabrication of a large-effective-area microstructured plastic optical fiber: design and transmission tests”, ECOC 2003. The fiber is made of plastic and comprises two shells of holes distributed over a hexagonal pattern, with the central hole and the inner shell of holes lacking. A large effective core radius reff is obtained with relative small hole-to-hole distance Λ. In particular, reff=1.44Λ. The Applicant observes that the fibers described by Asnaghi et al. is a multimode fiber unsuitable for long-distance optical telecommunications.
Further researches on microstructured fibers have demonstrated that reducing the number of holes can be disadvantageous in terms of transmission loss.
For example, T. P. White et al., ‘Confinement losses in microstructured optical fibers’, Optics Letters, vol 26, 2001, p. 1660-1662, and D. Ferrarini et al., ‘Leakage Losses in Photonic crystal Fibers’, F15, OFC 2003, vol. 2, p. 699-700, report that radiation losses are very high when the number of rings of holes in the cladding is small, since the transmitted radiation leaks away through a thin microstructured cladding. Both these articles show that the fiber performances in terms of attenuation are progressively improved passing from one ring to eight rings of holes. Also, in these articles the dependencies of the fiber loss on the cladding leakage as a function of Λ/λ (where Λ is the hole spacing and λ is the wavelength of the propagating light), d/Λ (where d is the hole diameter) and the number N of rings of holes in the cladding are discussed.
The Applicant is of the opinion that the quantitative results reported in the paper by T. P. White et al., referenced in the foregoing, are not particularly precise, because the electromagnetic field, which is a vectorial quantity, is there treated as a scalar quantity. Such a simplification is, according to the Applicant, of limited accuracy when large refractive index steps exist in the structure to be analyzed, such as those existing between the air in the holes and the background matrix at the hole surface. Furthermore, in the considered prior-art reference a fake material absorption in the background matrix was introduced, which could in turn limit the applicability of the method when very low losses such as those desirable for transmissive fibers are calculated.
Still according to the Applicant, a more precise, fully vectorial solution of the Maxwell equations is the one calculated in the above-cited article by D. Ferrarini et al. without the drawback of a fake background absorption.
More complex microstructured fibers are also described in the art. In the U.S. Pat. No. 5,802,236 a microstructured optical fiber is described comprising an inner cladding region having first cladding features, arranged in basically hexagonal form, surrounded by an outer cladding region comprising at least four layers of second cladding features. Two practical examples of fiber are discussed in that patent: in a first example, seven layers of features (cladding voids) are arranged around a central defect with a triangular unit cell, with a features array pitch of 2 μm; in a second example, the core is a silica rod surrounded by an inner layer of six features, which is in turn surrounded by more than four layers of smaller features, with a feature pitch of 0.925 μm. The Applicant observes that the large number of features involved in the fibers described in this patent poses the previously-mentioned loss problems.
The International patent application WO 02/39159 describes a microstructured optical fiber having a specially designed cladding to provide single mode waveguidance and low sensitivity to bending losses. The optical fiber has an inner and an outer cladding each comprising elongated features. The inner cladding features have normalized dimensions in the range from 0.35 to 0.50 and the outer cladding features have normalized dimensions in the range from 0.5 to 0.9, where the normalization factor is a typical feature spacing. The fiber is further characterized by a feature spacing of the inner cladding larger than 2.0 micron.
The Applicant observes that the fibers described in WO 02/39159 require different d/Λ for inner and outer cladding holes, and the same remark given before holds for these fibers.
In view of the state of the art outlined in the foregoing, it would therefore be desirable to provide a microstructured optical fiber suitable for long-haul transmission systems, in particular a single-mode microstructured optical fiber having very low transmission losses. According to the Applicant, very low losses are achievable only if all the possible attenuation contributions are considered in the fiber design, including losses due to contamination and imperfection in the holes and losses due to leakage through the cladding.
It would also be desirable to provide one such fiber that can be easily manufactured, which exhibits a reasonable manufacturing robustness, and which guarantees the above optical performances regardless of small geometrical imperfections which always result in the manufacturing, particularly as far as the dimensions and the regularity of the microstructures are concerned.